1. Field of the Invention
This invention relates to an apparatus for monitoring electric power systems and, more particularly, to an apparatus for digitally capturing a portion of a waveform of an electric power system in response to a waveform disturbance trigger condition.
2. Background Information
State of the art monitors for alternating current (AC) power systems incorporate microcomputers for calculating various electrical parameters such as RMS currents and voltages, peak currents and voltages, power, energy, power factor and the like. One such monitor is disclosed in U.S. Pat. No. 5,587,917. In addition to monitoring the various electrical parameters of the AC system, this monitor also digitally captures portions of the waveform for harmonic analysis. This places a very high burden on the microcomputer in the monitor. Accordingly, this monitor uses a slow sampling rate for gathering the data needed for performing the monitoring functions and operates at a second, higher rate for waveform capture in response to a manual command or automatically in response to values of selected ones of the electrical parameters which exceed predetermined thresholds.
A monitor/analyzer for an AC electrical system, disclosed in commonly owned U.S. patent application Ser. No. 08/608,387, filed on Feb. 28, 1996, responds to selected events by generating trigger signals when selected monitored electrical parameters, including neutral current, exceed programmed thresholds. The trigger signals initiate simultaneous waveform capture of multiple waveforms in the AC system. Multiple trigger signals result in sequential waveform capture so that if a plurality of triggers are programmed for the same event, extended waveform data is captured for the event.
Commonly owned U.S. patent application Ser. No. 08/608,386, filed on Feb. 28, 1996, discloses a monitor for an AC electrical system which generates on-line a display presenting the values, either in magnitude or as a percent of the fundamental, of fifty harmonics of any of the currents and voltages in the system.
A circuit interrupter, disclosed in commonly owned U.S. patent application Ser. No. 08/342,208, filed on Nov. 18, 1994, provides waveform capture in addition to protection functions. The circuit interrupter microcomputer operates in a protection mode to provide the protection functions and in a waveform capture mode to record the waveform data.
It is also known to employ a rate of change of current, di/dt, in tripping a circuit breaker. See, for example, U.S. Pat. Nos. 3,673,455; and 5,224,006.
Known monitors for AC electrical systems detect disturbances in such systems by employing information obtained from plural AC cycles.
There is a need, therefore, for an improved monitor for AC electrical systems which provides an improved capability for monitoring sub-cycle disturbances in such systems.